KMSAN: uninit-value in capi_write

[syzbot]

Hello,

syzbot found the following crash on:

HEAD commit:    11587f6ee534 kmsan: remove pr_err
git tree:       kmsan
console output: https://syzkaller.appspot.com/x/log.txt?x=120bd84f400000
kernel config:  https://syzkaller.appspot.com/x/.config?x=c8a62a4eb8ea3e9f
dashboard link: https://syzkaller.appspot.com/bug?extid=0849c524d9c634f5ae66
compiler:       clang version 8.0.0 (trunk 350161)
syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=14c53cd3400000
C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=13e2794b400000

IMPORTANT: if you fix the bug, please add the following tag to the commit:
Reported-by: syzbot+0849c524d9c634f5ae66@xxxxxxxxxxxxxxxxxxxxxxxxx

==================================================================
BUG: KMSAN: uninit-value in capi_write+0x791/0xa90  
drivers/isdn/capi/capi.c:700
CPU: 0 PID: 10025 Comm: syz-executor379 Not tainted 4.20.0-rc7+ #2
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS  
Google 01/01/2011
Call Trace:
 __dump_stack lib/dump_stack.c:77 [inline]
 dump_stack+0x173/0x1d0 lib/dump_stack.c:113
 kmsan_report+0x12e/0x2a0 mm/kmsan/kmsan.c:613
 __msan_warning+0x82/0xf0 mm/kmsan/kmsan_instr.c:313
 capi_write+0x791/0xa90 drivers/isdn/capi/capi.c:700
 do_loop_readv_writev fs/read_write.c:703 [inline]
 do_iter_write+0x83e/0xd80 fs/read_write.c:961
 vfs_writev fs/read_write.c:1004 [inline]
 do_writev+0x397/0x840 fs/read_write.c:1039
 __do_sys_writev fs/read_write.c:1112 [inline]
 __se_sys_writev+0x9b/0xb0 fs/read_write.c:1109
 __x64_sys_writev+0x4a/0x70 fs/read_write.c:1109
 do_syscall_64+0xbc/0xf0 arch/x86/entry/common.c:291
 entry_SYSCALL_64_after_hwframe+0x63/0xe7
RIP: 0033:0x440079
Code: 18 89 d0 c3 66 2e 0f 1f 84 00 00 00 00 00 0f 1f 00 48 89 f8 48 89 f7  
48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 0f 05 <48> 3d 01 f0 ff  
ff 0f 83 fb 13 fc ff c3 66 2e 0f 1f 84 00 00 00 00
RSP: 002b:00007ffd84911358 EFLAGS: 00000207 ORIG_RAX: 0000000000000014
RAX: ffffffffffffffda RBX: 00000000004002c8 RCX: 0000000000440079
RDX: 0000000000000001 RSI: 0000000020000180 RDI: 0000000000000003
RBP: 00000000006ca018 R08: 00000000004002c8 R09: 00000000004002c8
R10: 0000000000000000 R11: 0000000000000207 R12: 0000000000401900
R13: 0000000000401990 R14: 0000000000000000 R15: 0000000000000000

Uninit was created at:
 kmsan_save_stack_with_flags mm/kmsan/kmsan.c:204 [inline]
 kmsan_internal_poison_shadow+0x92/0x150 mm/kmsan/kmsan.c:158
 kmsan_kmalloc+0xa6/0x130 mm/kmsan/kmsan_hooks.c:176
 kmsan_slab_alloc+0xe/0x10 mm/kmsan/kmsan_hooks.c:185
 slab_post_alloc_hook mm/slab.h:446 [inline]
 slab_alloc_node mm/slub.c:2759 [inline]
 __kmalloc_node_track_caller+0xe18/0x1030 mm/slub.c:4383
 __kmalloc_reserve net/core/skbuff.c:137 [inline]
 __alloc_skb+0x309/0xa20 net/core/skbuff.c:205
 alloc_skb include/linux/skbuff.h:998 [inline]
 capi_write+0x12f/0xa90 drivers/isdn/capi/capi.c:691
 do_loop_readv_writev fs/read_write.c:703 [inline]
 do_iter_write+0x83e/0xd80 fs/read_write.c:961
 vfs_writev fs/read_write.c:1004 [inline]
 do_writev+0x397/0x840 fs/read_write.c:1039
 __do_sys_writev fs/read_write.c:1112 [inline]
 __se_sys_writev+0x9b/0xb0 fs/read_write.c:1109
 __x64_sys_writev+0x4a/0x70 fs/read_write.c:1109
 do_syscall_64+0xbc/0xf0 arch/x86/entry/common.c:291
 entry_SYSCALL_64_after_hwframe+0x63/0xe7
==================================================================


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